In classical mechanics, a particle is an eternal smallest element. It is a brick that builds everything that exists. Particles are the building blocks of any physical system. A particle has no size, no internal structure, but has some information attached. Typically, the information that comes with the particle is the mass, \label{word: mass}, but it can also include other observables, as the electric charge\label{word: charge}, the spin\label{word: spin}, or additional internal charges. By eternal, we mean that a particle cannot be created or destroyed. A particle exists.